Investigation of the Combustion Instability-NOx Tradeoff in a Dual Fuel Reactivity Controlled Compression Ignition (RCCI) Engine

The tradeoff between NOx emissions and combustion instability in an engine operating in the dual-fuel Reactivity Controlled Compression Ignition (RCCI) combustion mode was investigated using a combination of engine experiments and detailed CFD modeling. Experiments were performed on a single cylinder version of a General Motors/Fiat JTD 1.9L four-cylinder diesel engine. Gasoline was injected far upstream of the intake valve using an air assisted injector and fuel vaporization system and diesel was injected directly into the cylinder using a common rail injector. The timing of the diesel injection was swept from −70° ATDC to −20° ATDC while the gasoline percentage was adjusted to hold the average combustion phasing (CA50) and load (IMEPg) constant at 0.5° ATDC and 7 bar, respectively. At each operating point the variation in IMEP, peak PRR, and CA50 was calculated from the measured cylinder pressure trace and NOx, CO, soot and UHC were recorded. It was observed that a late injection strategy with start-of-injection (SOI) timings ranging from −30° ATDC to −20° ATDC could significantly reduce cycle-to-cycle variation with only a marginal increase in NOx emissions. To explain the sources of increased combustion stability, detailed computational fluid dynamics (CFD) simulations were used. The CFD simulations confirmed that the late injection timing produced a higher equivalence ratio ignition site that is less sensitive to fluctuations. By optimizing injection timing large improvements in combustion instability can be made while still maintaining low temperature combustion-like emissions.